Conventionally, vehicle-mountable junction boxes are available in, for example, a busbar system including internal wires in the form of busbars and a printed circuit board system including internal wires in the form of a printed circuit board. Over a vehicle-mountable junction box of the busbar system, a vehicle-mountable junction box of the printed circuit board system has advantages of, for example, being more easily changed in design or the like in accordance with the type, grade or shipping destination of the vehicle and allowing a circuit pattern to be formed in a shorter period of time.
However, a printed circuit board including a board formed of general glass epoxy has a problem of being liable to be raised in temperature because heat from electronic components or the circuit patterns mounted thereon is insulated by an insulating layer and thus the heat is concentrated on the circuit patterns. In addition, because the heat generated from one of the circuits is isolated from the heat generated in another circuit on the printed circuit board, the heat cannot be collected and radiate collectively.
In order to deal with this problem, it is known to use a metal core plate such that the heat from the electronic components or the circuit patterns is uniformized by the core plate and the heat is urged to radiate from the entirety of the board.
However, merely changing the material of the board and adopting a metal core board does not reduce the size or improve the mounting efficiency sufficiently.
In the meantime, Patent Document 1 discloses a metal core board which allows a plurality of circuits to be formed thereon, although this metal core board is not for a vehicle-mountable junction box.
This metal core board is formed as follows. In order to form a quadrangular independent island in a metal core (core plate), substantially U-shaped holes are punched in the metal core while two connection parts are left unpunched. Insulating plates are stacked on both of two surfaces of the metal core. Then, bores are formed from outside to remove the connection parts. As a result of the connection parts being removed, the island becomes independent from an area surrounding the island, namely, the metal core is divided into a plurality of areas.
The punched holes have a width sufficient to block the heat transmission to the island from the surrounding area.
Therefore, there are functions and effects that a plurality of circuits are formed and also the heat transmission is blocked by the presence of the punched holes in the metal core.
However, no metal core board for a vehicle-mountable junction box has ever adopted the structure of dividing the core as disclosed in Patent Document 1.
Automobiles are desired to have more functions and a certain size of space of a compartment. Especially from the viewpoint of providing a certain size of space of a compartment, the junction box is occasionally mounted in a high-temperature engine room and thus is required to be durable. However, for the metal core board disclosed in Patent Document 1, no consideration is given to warp, stress or the like which is caused to the core plate during or after the formation of the punched holes or the formation of the bores to remove the connection parts. When the core plate is warped or stressed, the core plate is distorted after being used for a long time, which may hinder the functions of the metal core board.
Therefore, even when the technology disclosed in Patent Document 1 is merely adopted for the metal core board for a vehicle-mounted junction box, the metal core board is distorted by the warp or stress caused to the core plate and does not have a necessary level of durability.
In addition, in the case of the metal core board disclosed in Patent Document 1, the separate island is formed merely for the purpose of heat radiation.
The bores made at the connection parts in order to divide the core are left open and are not used for anything.
Since the bores are left open, there is the following inconveniences. When, for example, soldering of a flow system is conducted in order to secure the electronic components mounted on the metal core board, the solder sprouts from the bores and adheres to areas where the solder should not adhere.
In addition, end surfaces of the metal core are exposed to an inner circumferential wall of each bore. This may undesirably cause core-to-core leaks when the plurality of circuits are used.
The punched holes formed in the metal core of Patent Document 1 are for blocking the heat transmission and so are wide.
Therefore, heat generated in one circuit is isolated from heat generated in another circuit. Namely, the heat does not radiate as being dispersed and uniformized in the entire metal core board, and therefore a local temperature rise cannot be suppressed. Accordingly, even when the technology disclosed in Patent Document 1 is merely adopted for the metal core board for a vehicle-mounted junction box, a necessary level of heat radiation effect is not provided.
Since the punched holes are wide, it cannot be expected to reduce the size or improve the mounting efficiency.
However, as the punched holes are narrower, or as the straight portions of the punched holes are longer, the punched holes act more like bent lines. As a result, the metal core is more liable to be warped or stressed. When the metal core is warped or stressed, the metal core is distorted after being used for a long time, which may hinder the functions of the metal core board.
The metal core is liable to be warped or stressed when being handled and also when being processed. A punched hole may be formed by use of a rooter, etching, drilling, punching by use of a die or the like. Punching by use of a die is most appropriate from the viewpoint of ease of work because punching can make a hole in one step of action. However, in the case of punching a hole by use of a die, as the resultant hole is thinner and longer, the core plate is more liable to be warped or stressed.
In addition, for punching a thin hole, the die needs to be have a thin blade. Such a blade may occasionally be stuck between a die block and a stripper, may fall down or may be bent, at the time of punching. This significantly wears or damages the die, and shortens the life of the die. As can be seen, there are inconveniences that, for example, the work is difficult, the working efficiency is low, and the cost is high.